The present invention is directed to nanostructures. More particularly, the invention provides arrays of long nanostructures in semiconductor substrates and methods thereof. Merely by way of example, the invention has been applied to arrays of long nanowires in silicon with certain thermoelectric properties. However, it would be recognized that the invention has a much broader range of applicability, including but not limited to use in solar power, battery electrodes and/or energy storage, catalysis, and/or light emitting diodes.
Thermoelectric materials are ones that, in the solid state and with no moving parts, can, for example, convert an appreciable amount of thermal energy into electricity in an applied temperature gradient (e.g., the Seebeck effect) or pump heat in an applied electric field (e.g., the Peltier effect). The applications for solid-state heat engines are numerous, including the generation of electricity from various heat sources whether primary or waste, as well as the cooling of spaces or objects such as microchips and sensors. Interest in the use of thermoelectric devices that comprise thermoelectric materials has grown in recent years in part due to advances in nano-structured materials with enhanced thermoelectric performance (e.g., efficiency, power density, or “thermoelectric figure of merit” ZT, where ZT is equal to S2 σ/k and S is the Seebeck coefficient, σ the electrical conductivity, and k the thermal conductivity of the thermoelectric material) and also due to the heightened need both for systems that either recover waste heat as electricity to improve energy efficiency or cool integrated circuits to improve their performance.
To date, thermoelectrics have had limited commercial applicability due to the poor cost performance of these devices compared to other technologies that accomplish similar means of energy generation or refrigeration. Where other technologies usually are not as suitable as thermoelectrics for use in lightweight and low footprint applications, thermoelectrics often have nonetheless been limited by their prohibitively high costs. Important in realizing the usefulness of thermoelectrics in commercial applications is the manufacturability of devices that comprise high-performance thermoelectric materials (e.g., modules). These modules are preferably produced in such a way that ensures, for example, maximum performance at minimum cost.
The thermoelectric materials in presently available commercial thermoelectric modules are generally comprised of bismuth telluride or lead telluride, which are both toxic, difficult to manufacture with, and expensive to procure and process. With a strong present need for both alternative energy production and microscale cooling capabilities, the driving force for highly manufacturable, low cost, high performance thermoelectrics is growing.
Thermoelectric devices are often divided into thermoelectric legs made by conventional thermoelectric materials such as Bi2Te3 and PbTe, contacted electrically, and assembled in a refrigeration (e.g., Peltier) or energy conversion (e.g., Seebeck) device. This often involves bonding the thermoelectric legs to metal contacts in a configuration that allows a series-configured electrical connection while providing a thermally parallel configuration, so as to establish a temperature gradient across all the legs simultaneously. However, many drawbacks may exist in the production of conventional thermoelectric devices. For example, costs associated with processing and assembling the thermoelectric legs made externally is often high. The conventional processing or assembling method usually makes it difficult to manufacture compact thermoelectric devices needed for many thermoelectric applications. Conventional thermoelectric materials are usually toxic and expensive.
Nanostructures often refer to structures that have at least one structural dimension measured on the nanoscale (e.g., between 0.1 nm and 1000 nm). For example, a nanowire is characterized as having a cross-sectional area that has a distance across that is measured on the nanoscale, even though the nanowire may be considerably longer in length. In another example, a nanotube, or hollow nanowire, is characterized by having a wall thickness and total cross-sectional area that has a distance across that is measured on the nanoscale, even though the nanotube may be considerably longer in length. In yet another example, a nanohole is characterized as a void having a cross-sectional area that has a distance across that is measured on the nanoscale, even though the nanohole may be considerably longer in depth. In yet another example, a nanomesh is an array, sometimes interlinked, including a plurality of other nanostructures such as nanowires, nanotubes, and/or nanoholes.
Nanostructures have shown promise for improving thermoelectric performance. The creation of 0D, 1D, or 2D nanostructures from a thermoelectric material may improve the thermoelectric power generation or cooling efficiency of that material in some instances, and sometimes very significantly (a factor of 100 or greater) in other instances. However, many limitations exist in terms of alignment and scale for the nanostructures needed in an actual macroscopic thermoelectric device comprising many nanostructures. The ability to process nanostructures in similar methods to the processing of silicon and other semiconductors would have tremendous cost advantages.
Hence, it is highly desirable to improve techniques for the formation of large arrays of very long nanostructures, such as nanowires or nanoholes, using less expensive and less toxic materials such as silicon, its alloys, and other suitable semiconductors. It is also highly desirable to form these large arrays of very long nanostructures from materials with advantageous electrical and thermal properties for use in thermoelectric devices.